Paper has either a matted or a high gloss surface finish depending on the quality of print desired. In either case, adhesion and dry time of an applied ink from an inkjet printing device are functions of the porosity of the paper and the surface coating. In substrates having matte finishes, pigments used are the primary factor in determining ink acceptance, dry time, and dry image properties. In substrates having glossy surfaces, however, the ink acceptance and image properties are generally more dependent on the resins used in the base coating. The coatings can be water- or solvent based. Either chemistry can provide the desirable optical qualities and enhance the appearance of the paper. When printing from a laser printing device, on the other hand, a toner is fused to the media at a temperature of about 350 degrees F. to about 400 degrees F. and at a pressure of about 70 pounds per square inch (psi) to about 140 psi. Thus, the media used for laser printing softens enough at the elevated temperature at which the toner fuses to the media to allow the toner to anchor at the surface coating while not adhering to the rollers or other printer mechanisms and causing the printing device to not operate properly.
Printing on holographic films or metallic foils has often required the application of a transparent base coat in order to maintain the underlying image appearance. The development of optically clear water-based ink receptive coatings for films and foils involves several additional considerations. First, the film or foil substrate is generally not porous or is porous only to an insignificant amount. A lack of sufficient porosity thus may not provide a suitable degree of adhesion of the coating to the film or foil and furthermore may adversely affect the drying rate of the ink. To facilitate the adhesion of the coating, a tie coat is generally applied to the topmost layer of the film or foil to provide a surface to which subsequent coatings can be anchored. For example, in U.S. Pat. No. 6,458,449 to Parent et al. an ink jet printable coating for holographic paper utilizes a primer layer having a surface tension that is preferably in the range of 35 to 80 dynes and a secondary layer. The secondary layer includes a plurality of microscopic pores sized to allow ink jet ink to penetrate during the printing process.
Second, metallic substrates are often primed with protective coatings to prevent oxidation of the metal, which can detract from the adhesion of subsequent coating layers. Protective clear coats can be difficult to wet out and adhere to when over-coated with water-based coating systems. The ink receptive coating is therefore oftentimes required to fulfill a variety of functions including adhesion to the substrate, ink acceptance and drying, high clarity to highlight the substrate, and good application properties. Since no one vehicle provides all of the desired properties of clear coatings on foils, conventional practice typically results in the use of multiple layer coatings to achieve desired results. This approach limits production application to multiple head coating machines or to the transfer of product between machines to apply the required number of coats. Multiple coats further tend to reduce film clarity due to different indices of refraction between layers. Film clarity may also be reduced as a result of irregularities at the interfaces of each layer. The use of multiple coats and coating machines further increases the potential for product quality issues related to the physical handling of the films or foils during multiple applications of the liquid coating.
A need exists for a single layer water-based optically clear receptive coating that will adhere to protective coats when applied to a metal foil laminated paper or a holographic film for the acceptance of an ink from an ink-jet printing device. A need also exists for an optically clear single layer water-based coating usable on holographic and metallic media that will accept both ink jet and laser printing to provide a dual use media.